Hi aehome,
IFD and ILD are basically the same thing. One stands for Indentation force deflection and the other one for Indentation load deflection. IFD is more commonly used for polyfoam and ILD for latex (if the latex has an ILD rating because some only go by density). Dunlop also has a higher compression modulus than Talalay (gets firmer faster when you sink into it) so a 2" dunlop topper over a firm Dunlop core would be quite firm for most people. I think your “analysis” is probably accurate and shows a good basic understanding of the many interacting factors involved in pressure relief and alignment. I would also be aware that the 5.3 lb 100% natural Dunlop sold by Absolutecomfortonsale may be a little firmer than they indicate (you can see some approximate density / ILD comparisons for 100% natural Dunlop in post #2 here).
The current construction you have would be more of a progressive construction than differential already (your comfort layer would need to “borrow” from the layer below it for most people because it is only 2" thick) but they are really just concepts rather than specific constructions so there would be an argument that it was either depending on how it interacts with you. The idea of a differential construction is that the comfort layer is thick and soft enough all by itself that it doesn’t need any “help” from the middle layers for pressure relief and can go directly on top of a much firmer layer with a much firmer ILD or (differential) and is thick enough so you don’t feel the “transition” between the layers. Once you have a reference point though then making changes would be based more on your actual experience and “symptoms” which would be more important than which type of design it is closest to.
I should clarify as well that I would consider your 2" latex layer would be a topper or “layer” rather than a “mattress pad” (just to save me the confusion of reading “mattress pad” and being unsure what you are referring to). A mattress pad typically refers to a thinner pad and is made of a fiber like wool, cotton, or a synthetic (like a down alternative although once again a thick mattress pad could be similar to a thin topper because there is no specific definition for either.
There isn’t a “formula” that you can use for choosing a topper because everyone is different but there are some guidelines in post #2 here that may be helpful. The choice between memory foam and latex is strictly one of preference. I would keep in mind that your Dunlop top layer is fairly firm so you may want to add a softer layer on top and use it for the transition layer although if your Dunlop topper is soft enough or if you prefer a firmer surface feel or being more “on the mattress” than “in the mattress” then latex over memory foam also has a rather unique feel that some people like (including me) with a firmer more resilient sleeping surface with some slower response and sinking in underneath it. If the layers on top of the memory foam are too thick and insulating or if the memory foam is too temperature sensitive then it may remain too firm to provide good pressure relief so it’s more traditional and common and possible “safer” to use the softer foam on top though which would allow you to sink in a little more although of course experimenting would be the only way to know which was best with the specific layers you had.
I would also only buy only one layer at a time instead of trying to deal with the much more complex and unpredictable variables involved with multiple layers that you haven’t tried. I would also tend towards materials that you are familiar with and have tested or slept on in person.
Phoenix